Backside illuminated photo-sensitive device with gradated buffer layer

ABSTRACT

A photo-sensitive device includes a uniform layer, a gradated buffer layer over the uniform layer, a silicon layer over the gradated buffer layer, a photo-sensitive light-sensing region in the uniform layer and the silicon layer, a device layer on the silicon layer, and a carrier wafer bonded to the device layer.

PRIORITY INFORMATION

The present application is a continuation of U.S. patent Ser. No.16/166,656, filed Oct. 22, 2018, which is a continuation of U.S. patentapplication Ser. No. 14/824,910, filed Aug. 12, 2015, which is adivisional of U.S. patent application Ser. No. 13/963,079, filed Aug. 9,2013, each of which is hereby incorporated by reference in its entirety.

BACKGROUND

Photo-sensitive devices are used in a variety of electronic devices. Forexample, an array of photo-sensitive devices can be used to form animage sensor array to be used in a digital camera. A photo-sensitivedevice is typically includes a light-sensing region within asemiconductor material that transfers energy from photons intoelectrical energy.

The light-sensing region is typically formed into a semiconductormaterial through an implantation process to form either a p-i-n junctionor a p-n junction. The semiconductor material in which the light-sensingregion is formed is usually partially made of germanium in addition tosilicon. This provides various benefits to the photo-sensitive device.

The efficiency at which the photo-sensitive device operates is affectedby the characteristics of the semiconductor material in which thelight-sensing region is formed. When using a semiconductor crystal thatis made of silicon germanium, there may be defects in the crystal due tothe different lattice constants between silicon and germanium. Thesedefects are sometimes referred to as dislocations. These defectsadversely affect the efficiency at which the photo-sensitive deviceoperates. Thus, it is desirable to reduce these defects as much aspossible.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1A-1E are diagrams showing an illustrative formation of a backsideilluminated photo-sensitive device with a gradated buffer layer,according to one example of principles described herein.

FIG. 2A is a diagram showing an illustrative optical grating structureformed into a uniform layer of the photo-sensitive device, according toone example of principles described herein.

FIG. 2B is a diagram showing an illustrative passivation layer formedinto a uniform layer of the photo-sensitive device, according to oneexample of principles described herein.

FIG. 3 is a flowchart showing an illustrative method for forming abackside illuminated photo-sensitive device with a gradated bufferlayer, according to one example of principles described herein.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the performance of a first process before a second process in thedescription that follows may include embodiments in which the secondprocess is performed immediately after the first process, and may alsoinclude embodiments in which additional processes may be performedbetween the first and second processes. Various features may bearbitrarily drawn in different scales for the sake of simplicity andclarity. Furthermore, the formation of a first feature over or on asecond feature in the description that follows may include embodimentsin which the first and second features are formed in direct contact, andmay also include embodiments in which additional features may be formedbetween the first and second features, such that the first and secondfeatures may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

FIGS. 1A-1E are diagrams showing an illustrative formation of a backsideilluminated photo-sensitive device 100 with a gradated buffer layer.According to certain illustrative examples, a sacrificial gradatedbuffer layer 104 is formed onto a sacrificial substrate 102. Theselayers are referred to as sacrificial because they will eventually beremoved as will be discussed in further detail below.

The sacrificial substrate 102 is made of a semiconductor material suchas silicon. The sacrificial substrate 102 is used to support anepitaxial growth process to form the gradated buffer layer 104. Anepitaxial process is one in which a semiconductor crystal is grown ontoan already existing semiconductor material. The epitaxially grown layeris typically formed through use of a gaseous precursor.

The sacrificial gradated buffer layer 104 is made of a semiconductormaterial that matches the substrate as well as a secondary material. Forexample, if the semiconductor substrate is made of silicon, then thesacrificial gradated buffer layer 104 is made of silicon and thesecondary material. The secondary material is based on the material inwhich the layer above the buffer layer 104 will be. For example, if theabove layer includes germanium, then the secondary material may begermanium.

The sacrificial gradated buffer layer 104 changes in concentration ofthe secondary material, which in this example is germanium, as it getsfarther from the sacrificial substrate 102. For example, theconcentration of germanium at the border of the sacrificial gradatedbuffer layer 104 and the sacrificial substrate 102 is zero. As thesacrificial gradated buffer layer 104 is grown, the concentration ofgermanium increases.

FIG. 1A illustrates a doping profile in association with the sacrificialsubstrate 102 and the sacrificial gradated buffer layer 104. Thehorizontal axis represents the concentration of the secondary material,which in this case germanium. The vertical axis represents the positionin along the depth of the photo-sensitive device 100. As illustrated,the doping profile 106 of the sacrificial gradated buffer layer 104increases in concentration as it moves farther away from the sacrificialsubstrate 102.

In one example, the gradient of the gradated buffer layer may be anincrease of 10% concentration for every 0.1 micrometers of thickness. Insome cases, the change in concentration may be within a range of about 5to 20 percent change in germanium concentration for every 0.1micrometers in thickness. The overall thickness of the sacrificialgradated buffer layer 104 may be within a range of about 0.1 to 1.0micrometers. Thus, if the thickness of an illustrative gradated bufferlayer is 0.4 micrometers and the gradient is a 10% change per 0.1micrometers, then the total germanium concentration at the top of thegradated buffer layer 104 would be approximately 40%.

In some examples, the gradient may be such that it changes from 0%germanium to 100% over the thickness of the layer 104. While the dopingprofile 106 is illustrated as a linear profile, the doping profile 106may be non-linear. Various methods of forming the epitaxially grown,gradated buffer layer may be used. For example, to reduce stress, thetemperature may be lower than would otherwise be used. For example, theprocess may be performed at a temperature ranging between about 400 and700 degrees Celsius.

FIG. 1B is a diagram showing an illustrative formation of a uniformlayer 108 on top of the sacrificial gradated buffer layer 104. Theuniform layer 108 maintains a steady concentration of throughout itsthickness. Thus, the doping profile 110 of the uniform layer 108manifests as a straight line. The uniform layer 108 may have a thicknesswithin a range of about 0.1-2.0 micrometers.

The uniform layer 108 may be made of a variety of materials. In oneexample, the uniform layer 108 may be made of Silicon Germanium (SiGe).In one example, the uniform layer may be made of pure germanium. In afurther example, the uniform layer may be a semiconductor materialhaving a quantum dot or quantum well matrix formed therein. In someexamples, a SiGe epitaxy process can allow in-situ doping of p-type orn-type dopants to reduce induced Silicon crystalline damages and improveCIS performance. These dopants may include Boron, Phosphorous or Carbon.The precursor gases to use such dopants in-situ are respectively B₂H₆,PH₃ and CH₃SiH₃.

In the example where the uniform layer 108 is made of silicon germanium,the concentration of germanium may match the concentration of germaniumat the top of the sacrificial buffer layer 104. Using the example wherethe concentration of germanium at the top of the layer is 40%, then theuniform layer 108 will maintain a concentration of 40% germaniumthroughout the thickness of the uniform layer 108.

In the example where the uniform layer 108 is made of pure germanium,the entire uniform layer 108 will be made of pure germanium. In thiscase, the sacrificial gradated buffer layer 104 may change from 0 to100% germanium concentration over its thickness. Thus, the germaniumconcentration at the top of the buffer layer 104 would match thegermanium concentration of the uniform layer 108, which in this case ispure germanium.

Forming the sacrificial graded buffer layer 104 in this manner allowsthe defects and dislocations resulting from forming one type of crystalon another type of crystal to be “trapped” in the buffer layer. Thus,the defects are mostly formed in an area that is ultimately to beremoved. If the uniform layer 108 is formed directly onto the siliconsubstrate with no buffer layer, then defects or dislocation would bemore prevalent throughout the entire uniform layer 108. This wouldadversely affect the efficiency of a photo-sensitive light-sensingregion 118 formed into the uniform layer 108.

In some examples, the uniform layer 108 may have quantum dots or aquantum well formed therein. As will be appreciated by those skilled inthe relevant art, quantum dots can be formed into semiconductormaterials to aid in the efficiency of photo-sensitive devices. Quantumdots are typically between 5 and 50 nm in size. Quantum dots may bedefined by lithographically patterned gate electrodes, or by etching ontwo-dimensional electron gases in semiconductor heterostructures. Aquantum dot matrix or quantum well matrix may be formed into either asilicon germanium uniform layer 108 or a pure germanium uniform layer108.

FIG. 1C is a diagram showing an illustrative formation of a secondgradated buffer layer 112. The second gradated buffer layer 112 is areverse gradient of the sacrificial gradated buffer layer 104. Asillustrated, the doping profile 114 of the second gradated buffer layer112 slopes back down to a concentration of zero.

The bottom of the second gradated buffer layer 112 may match thegermanium concentration layer of the uniform layer 108. Thisconcentration will slowly be reduced as the secondary graded bufferlayer 112 increases in height. Thus, in the example where the uniformlayer 108 is 40% germanium, the concentration of germanium at the bottomof the second gradated buffer layer 112 will be 40%. This concentrationwill be reduced as the height of the second gradated buffer layer 112increases. In the example where the uniform layer 108 is 100% germanium,the concentration of germanium at the bottom of the second gradatedbuffer layer 112 will be 100%. This concentration will also be reducedas the height of the second gradated buffer layer 112 increases.

The doping profile 114 of the second gradated buffer layer 112 does notnecessarily have to be a mirror image of the doping profile 106 of thesacrificial gradated buffer layer 104. The doping profile 114 isillustrated as a linear profile. However, the doping profile 114 mayalso be non-linear. The thickness of the second gradated buffer layer112 may also be within a range of about 0.01 to 0.5 micrometers.

FIG. 1D is a diagram showing an illustrative formation of a puresemiconductor layer 116 on top of the second gradated buffer layer. Thepure semiconductor layer 116 is made of the material other than thesecondary material of the second gradated buffer layer 112. For example,if the secondary material is germanium, and the remaining material ofthe second buffer layer 112 is silicon, then the pure semiconductorlayer 116 is also made of silicon. The pure semiconductor layer 116 mayhave a thickness within a range of about 0.01 to 1.0 micrometers.

FIG. 1D also illustrates the formation of a light-sensing region 118that extends over the uniform layer 108, the second gradated bufferlayer 112, and part of the pure semiconductor layer 116. Thelight-sensing region 118 may be formed through implantation. Thelight-sensing region 118 may be a p-n junction or a p-i-n junction.Various other components may be formed into the semiconductor layer 116that connect the light-sensing region 118 to other components such astransistors and other circuit elements that are used to operate inconcordance with the light-sensing region 118.

FIG. 1E illustrates the formation of a device layer 120 and a carrierwafer 122 onto the pure semiconductor layer 118. The device layer 120 isdisposed onto the silicon layer 116. Disposing the device layer 120 mayinclude either forming the device layer on the silicon layer 116.Alternatively, disposing the device layer 120 may include forming thedevice layer 120 separately and then bonding it to the silicon layer116. With these layers in place, the sacrificial layers 102, 104 can beremoved by a removal process 124. Thus, the backside of thelight-sensing region 118 is exposed and can sense light being projectedonto the backside.

The device layer is fabricated by semiconductor process before or afterphoto-diode formation. The device layer 120 may include various circuitcomponents and metal interconnects that connect to the light-sensingregion 118. For example, in an image sensor array, each light-sensingregion 118 is addressed through metal interconnects. Each light-sensingregion 118 in the array may correspond to a pixel within an image formedfrom the image sensor array. Each light-sensing region 118 may thus beassociated with specific circuit elements such as transistors to measurethe intensity of light at specified wavelengths that impinge upon thatlight-sensing region 118.

The circuitry within the device layer may be formed using standardphotolithographic techniques. For example, a photolithography processmay include processing steps of photoresist coating, soft baking, maskaligning, exposing, post-exposure baking, developing photoresist andhard baking. The lithography process may implement krypton fluoride(KrF) excimer lasers, argon fluoride (ArF) excimer lasers, ArF immersionlithography, extreme ultra-violet (EUV) or electron-beam writing(e-beam). The photolithography exposing process may also be implementedor replaced by other proper methods such as maskless photolithography,ion-beam writing, and molecular imprint. When applying the developingsolution to the exposed photoresist layer, the sacrificial layerunderlying the exposed photoresist region (for positive photoresist) ispartially or completely removed as well.

In order to remove the sacrificial layers, the device layer 120 isbonded to a carrier wafer 122. This is because the device layer 120 andall layers below it may not be structurally sufficient to withstand theremoval process 124. The removal process may be an etching process or achemical mechanical polishing (CMP) process. The removal process removesboth the sacrificial substrate 102 and the sacrificial gradated bufferlayer 104. Thus, all the defects and dislocations resulting from thegradual increase in germanium concentration are removed by the removalprocess 124. This leaves a uniform layer 108 with relatively few defectsor an extraordinarily low defect density. In some cases, a low defectdensity is defined and the formation of the uniform layer is designed toproduce a layer having defects below that threshold using principlesdescribed herein. Thus, the light-sensing region 118 formed therein canoperate more effectively.

FIG. 2A is a diagram showing an illustrative optical grating structure202 formed into a uniform layer of the photo-sensitive device. Accordingto certain illustrative examples, an optical grating structure may beformed through use of the quantum dot or quantum well matrix. Theoptical grating can be formed during the formation of the uniform layer108. The optical grating 202 layer maybe used to filter light to aspecific frequency range. For example, the optical grating can bedesigned to allow “red” light or red-shift light (red-shift meanswavelength shifts to longer wavelength) to pass while blocking othercolors of light. Thus, the light-sensing region 118 can be used tomeasure the intensity of “red” light.

FIG. 2B is a diagram showing an illustrative passivation layer 204formed into a uniform layer of the photo-sensitive device. According tocertain illustrative examples, the passivation layer 204 may be formedusing a P+ type material such as Boron. The passivation layer may beformed during formation of the uniform layer 108. This may be done byintroducing B2H6 into the epitaxial growth process. Thus, thepassivation layer is formed in-situ. In addition, in-situ doped Carboncan further cause retardation boron diffusion after the post-epitaxyannealing process.

The passivation layer or P+ type layer 204 may have a thickness andconc. within a range of 10-100 nanometers and 10¹⁸-10²¹ cm-3. The P+type layer may be made of Boron, for example. The purpose of thepassivation layer 204 is to protect the surface of the uniform layer 108from external contaminants or other things that may cause damage to theuniform layer 108. Moreover, a Boron-rich layer can create a diffusionbarrier to prevent interface recombination of photo-induced electrons.

FIG. 3 is a flowchart showing an illustrative method for forming abackside illuminated photo-sensitive device with a gradated bufferlayer. According to certain illustrative example, a method 300 forforming 302 a backside illuminated photo-sensitive device includes astep for forming a gradated sacrificial buffer layer onto a sacrificialsubstrate. The method further includes a step for forming 304 a uniformlayer onto the gradated sacrificial buffer layer. The method furtherincludes a step for forming 306 a second gradated buffer layer onto theuniform layer. The method further includes a step for forming 308 asilicon layer onto the second gradated buffer layer. The method furtherincludes a step for bonding 310 a device layer to the second bufferlayer. Alternatively, the device layer may be formed 311 onto the secondgradated buffer layer. The method further includes a step for removing312 the gradated sacrificial buffer layer and the sacrificial substrate.

According to certain illustrative examples, a method for forming abackside illuminated photo-sensitive device includes forming a gradatedsacrificial buffer layer onto a sacrificial substrate, forming a uniformlayer onto the gradated sacrificial buffer layer, forming a secondgradated buffer layer onto the uniform layer, forming a silicon layeronto the second gradated buffer layer, disposing a device layer on thesecond buffer layer, and removing the gradated sacrificial buffer layerand the sacrificial substrate.

According to certain illustrative examples, a photo-sensitive deviceincludes a uniform layer a gradated buffer layer over the uniform layer,a silicon layer over the gradated buffer layer, a photo-sensitivelight-sensing region 118 in the uniform layer and the silicon layer, adevice layer on the silicon layer, and a carrier wafer bonded to thedevice layer. The defects of the uniform layer are below a definedthreshold.

According to certain illustrative examples, a method for forming abackside illuminated photo-sensitive device includes forming a gradatedsacrificial buffer layer onto a sacrificial substrate, the gradatedsacrificial buffer layer varying in germanium concentration. The methodfurther includes forming a uniform layer onto the gradated sacrificialbuffer layer, the uniform layer including an in-situ doped passivationlayer adjacent to the gradated sacrificial buffer layer. The methodfurther includes forming a second gradated buffer layer onto the uniformlayer, the second gradated buffer layer varying in germaniumconcentration. The method further includes forming a silicon layer ontothe second gradated buffer layer, disposing a device layer on the secondsilicon layer, forming a photo-sensitive light-sensing region 118 intothe uniform layer and the silicon layer, bonding a carrier wafer to thedevice layer, and removing the gradated sacrificial buffer layer and thesacrificial substrate.

It is understood that various different combinations of the above-listedembodiments and steps can be used in various sequences or in parallel,and there is no particular step that is critical or required.Additionally, although the term “electrode” is used herein, it will berecognized that the term includes the concept of an “electrode contact.”Furthermore, features illustrated and discussed above with respect tosome embodiments can be combined with features illustrated and discussedabove with respect to other embodiments. Accordingly, all suchmodifications are intended to be included within the scope of thisinvention.

The foregoing has outlined features of several embodiments. Those ofordinary skill in the art should appreciate that they may readily usethe present disclosure as a basis for designing or modifying otherprocesses and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those of ordinary skill in the art should also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A device comprising: a uniform layer thatincludes an optical grating structure, wherein the uniform layerincludes a dopant having a first concentration; a gradated buffer layerdisposed over the uniform layer, wherein the gradated buffer layerincludes the dopant having a second concentration that is different thanthe first concentration; a semiconductor material layer disposed overthe gradated buffer layer; a device layer disposed over thesemiconductor material layer; and a light-sensing region extending fromthe optical grating structure, through the uniform layer, and partiallyinto the semiconductor material layer, wherein the light-sensing regionhas a different light sensitivity than a portion of the uniform layerthat is without the light sensing region, wherein the optical gratingstructure includes a first edge and an opposing second edge, and whereinthe light-sensing region extends through the first edge of the opticalgrating structure and into the optical grating structure withoutextending to the second edge of the optical grating structure.
 2. Thedevice of claim 1, wherein the optical grating structure is a quantumdot matrix.
 3. The device of claim 1, wherein the optical gratingstructure is a quantum well matrix.
 4. The device of claim 1, whereinthe gradated buffer layer includes a first side facing the uniform layerand an opposing second side facing away from the uniform layer, andwherein the second concentration of the dopant within the gradatedbuffer layer decreases from the first side to the second side of thegradated buffer layer.
 5. The device of claim 1, wherein the uniformlayer physically contacts the gradated buffer layer, and wherein thegradated buffer layer physically contacts the semiconductor materiallayer.
 6. The device of claim 1, wherein the light-sensing regionincludes a p-n junction.
 7. The device of claim 1, wherein thelight-sensing region includes a p-i-n junction.
 8. The device of claim1, wherein the optical grating structure allows light having a firstfrequency to pass while blocking light having a second frequency that isdifferent that the first frequency.
 9. The device of claim 1, whereinthe semiconductor material layer includes a third edge and an opposingfourth edge, and wherein the light-sensing region extends through thethird edge of the semiconductor material layer and into thesemiconductor material layer without extending to the fourth edge of thesemiconductor material layer, wherein the third edge of thesemiconductor material layer is closer to the first edge of the opticalgrating structure than the fourth edge of the semiconductor materiallayer.
 10. A device comprising: a uniform semiconductor layer thatincludes an optical grating structure, wherein the uniform semiconductorlayer includes a dopant at a uniform concentration throughout theuniform semiconductor layer; a gradated buffer semiconductor layerdisposed over the uniform semiconductor layer, wherein the gradatedbuffer semiconductor layer includes the dopant at a varied concentrationthroughout the gradated buffer semiconductor layer; a semiconductorlayer disposed over the gradated buffer semiconductor layer; and aphoto-sensitive region disposed within the optical grating structure andextending through the gradated buffer semiconductor layer, wherein thephoto-sensitive region has a different light sensitivity than a portionof the optical grating structure that is without the photo-sensitiveregion, wherein the optical grating structure includes a first edge andan opposing second edge, and wherein the photo-sensitive region extendsthrough the first edge of the optical grating structure and into theoptical grating structure without extending to the second edge of theoptical grating structure.
 11. The device of claim 10, wherein theoptical grating structure includes a quantum dot matrix.
 12. The deviceof claim 10, wherein the optical grating structure blocks a first colorof light.
 13. The device of claim 10, wherein the dopant is germanium.14. The device of claim 10, further comprising a device layer disposedover the semiconductor layer.
 15. The device of claim 14, wherein thedevice layer includes circuit components and metal interconnects thatconnect to the light sensing region.
 16. A device comprising: a uniformlayer interfacing with an optical grating structure, the uniform layerincluding a material having a same concentration throughout a thicknessof the uniform layer; a gradated layer interfacing with the uniformlayer, the gradated layer including the material at variousconcentrations throughout the thickness of the gradated layer; asemiconductor layer interfacing with the gradated layer; and alight-sensing region disposed within the optical grating structure andextending through the uniform layer and the gradated layer, wherein thelight-sensing region has a different light sensitivity than a portion ofthe optical grating structure and a portion of the gradated layer thatdo not include the light-sensing region, wherein the optical gratingstructure includes a first edge and an opposing second edge, and whereinthe light-sensing region extends through the first edge of the opticalgrating structure and into the optical grating structure withoutextending to the second edge of the optical grating structure.
 17. Thedevice of claim 16, wherein the optical grating structure includes aquantum well matrix.
 18. The device of claim 16, wherein the materialincludes germanium.
 19. The device of claim 16, wherein the opticalgrating structure includes a quantum dot matrix.
 20. The device of claim16, where in the semiconductor layer is formed of silicon and is free ofgermanium.